added event hooks so users can monitor when angular-loading-bar is react...

Here's something I found useful. I use it to see when loading-bar starts showing progress for a specific request, so I can match loading icons for specific elements exactly with the loading bar.

I used it like this:

```javascript
app.run(['$rootScope', function($rootScope) {
   $rootScope.$on('cfpLoadingBar:loading:/service/fetchEmployees', function() {
      // hide employee editing controls and show loading icon
   });
   
   $rootScope.$on('cfpLoadingBar:loaded:/service/fetchEmployees', function() {
     // hide loading icon and show employee editing controls
   });
}])
```

Opinion?